Bearing performance is measured by bearing load capacity and stiffness. Load capacity denotes rotor support limit during operation. Stiffness denotes the restoring force imparted to a shaft by the bearing when the shaft is deflected from its geometric axis. High bearing stiffness is desirable in order to maintain accurate shaft positioning as loads are applied to the shaft.
Conventional passive magnetic bearings are noted for their high load capacity and stiffness. Such bearings are characterized by two sets of permanent magnets. One set of magnets is employed in the rotor, and the other set of magnets is employed in the stator. Repulsive forces between the two sets of magnets cause the shaft to be suspended. The shaft can be rotated by a minimal amount of force. See, e.g., Meeks U.S. Pat. No. 3,614,181 and Wasson U.S. Pat. No. 4,072,370.
According to Earnshaws Theorem, however, total permanent magnet levitation is inherently unstable since the force is related to the inverse square of the distance. As a consequence of this instability, conventional passive magnetic bearings are not practical for use in bearing systems.
High-temperature superconducting bearings are noted for long life, reliability and low parasitic bearing power loss. These passive bearings can be made of Type I and Type II superconducting materials. Type I superconductors have the ability to screen out all or some of the magnetic flux applied by an external source. When cooled below a critical temperature T.sub.c, Type I superconductors exhibit total flux expulsion for applied magnetic fields less than some critical field H.sub.c. This phenomenon is known as the "Meissner Effect." When expelled, the flux flows around the superconductor, providing a lifting force. This lifting force causes a magnet to be levitated above a Type I superconductor that is held stationary.
However, bearings made of Type I superconducting materials are thought to experience rotor stability problems. As with conventional passive magnetic bearings, bearings made of Type I superconducting materials are not practical for use in bearing systems.
Type II superconducting materials are more commonly used for rotating bearings. Type II superconductors also exhibit total flux expulsion for applied magnetic fields less than a first critical field H.sub.c1. For applied magnetic fields in excess of a second critical field H.sub.c2, the superconductivity is lost. In between critical fields H.sub.c1 and H.sub.c2, however, Type II superconductors exhibit partial flux exclusion. Partial flux exclusion is believed to be caused by inhomogeneities (e.g., pores, inclusions, grain boundaries) inside the Type II superconductor. When the magnetic field is being induced into the superconductor, the superconductor offers resistance to change or displacement of this induced magnetic field. Some of the magnetic flux lines become "pinned" within the superconducting material. This phenomenon is known as "flux-pinning." The remaining flux lines are repelled by the flux lines pinned in the superconductor. This repulsion causes levitation. Thus, levitation does not arise from the Meissner effect. Instead, levitation occurs because the superconductor behaves more like a perfect conductor than a Meissner conductor.
Due to its flux-pinning properties, the Type II superconducting material gives superconducting bearings a measure of stability. Thrust bearings can be created by levitating a magnet above a disk made of a Type II superconductor. See, e.g., Agarwala U.S. Pat. No. 4,892,863. Journal bearings can be created by levitating a cylindrical magnet inside a hollow cylinder made of Type II superconducting material. See, e.g., Gyorgy et al. U.S. Pat. No. 4,797,386.
High-temperature superconducting bearings are ideal for use in aerospace turbomachinery applications where long life, reliability, and low parasitic bearing power loss are required. However, bearings made of Type II superconducting material have only the levitation force (load capacity) and rotor equilibrium restoration force (stiffness) for applications requiring very low load capacity and stiffness.
Therefore, it is an object of the present invention to provide a passive bearing that has the strength and machinability of a composite material, the load capacity and stiffness of a permanent magnet and the stability of a Type II superconducting material.